[clangd] Get rid of unnecessary source transformations in locateMacroAt

Summary:
All callers are already passing spelling locations to locateMacroAt.
Also there's no point at looking at macro expansion for figuring out undefs as
it is forbidden to have PP directives inside macro bodies.
Also fixes a bug when the previous sourcelocation is unavailable.

Reviewers: sammccall, hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D75259